1. Field of the Invention
The present invention relates to an intracavity wavelength conversion solid-state laser generator, and particularly relates a solid-state laser generator that converts the wavelength in two or more stages using a plurality of nonlinear optical crystals.
2. Description of the Related Art
Nd:YAG lasers or other solid-state laser generators are widely used as machining lasers. Recently, solid-state laser generators are rapidly increasing in output from several hundred watts to several kilowatts, and the field of application of the lasers is accordingly expanding from the conventional microfabrication field to welding and cutting applications in the automotive industry. Nevertheless, most solid-state laser generators have an emission wavelength range that is limited to the near infrared range of about 1 μm, bringing about a drawback in that the reflectivity of wavelengths outside the near infrared range is high, and the machining efficiency is poor in copper, silicon, and some other materials that have a low absorption rate.
For this reason, methods have been proposed in the prior art in which LiB3O5 (lithium triborate, LBO), KTiOPO4 (KTP), β-BaB2O4 (barium borate, BBO), and other nonlinear optical crystals are used to convert the emission wavelength to a shorter wavelength, i.e., a second harmonic and a third harmonic, further to a fourth and fifth harmonic, and so on, to reduce the reflectivity in the surface of the workpiece, to increase laser light absorption, and to thereby increase the machining efficiency. Extracavity wavelength conversion and intracavity wavelength conversion are types of wavelength conversion in which such nonlinear optical crystals are used.
External cavity wavelength conversion has a low conversion rate from a laser light at a fundamental wavelength (hereinafter referred to as fundamental laser light) to a harmonic laser light, and fundamental laser light must be condensed with high power density in the nonlinear optical crystals in order to obtain a high conversion rate. For this reason, the upper limit of the output obtained from a simple resonator is in the over 100-watt category even with second-harmonic laser light, and it is difficult to achieve a higher output when reliability is considered. With wavelength conversion of a third harmonic, fourth harmonic, and higher harmonics, the output is about 50 W at best because the resulting second harmonic is used.
Intracavity wavelength conversion has a high conversion rate to second-harmonic laser light and better reliability in comparison with external cavity wavelength conversion. This method is disadvantageous, however, in that the thermal lens effect generated in the solid-state laser medium does not allow higher output to be obtained while a stable resonant condition is maintained. The thermal lens effect is a phenomenon in which the solid-state laser medium is heated by being excited, the temperature distribution produced inside the solid-state laser medium creates a refractive index distribution, and the solid-state laser medium behaves like a lens.
Common solid-state laser media absorb almost no fundamental laser light, but often have high absorption characteristics in relation to laser light whose wavelength has been converted, particularly laser light that has been converted to a shorter wavelength. In view of the above, solid-state laser generators having intracavity wavelength conversion are configured to bend the optical path of the laser light at least once and to separate the optical path into wavelength-converted laser light and fundamental laser light by using a dielectric multilayer film mirror to extract only the wavelength-converted laser light from the resonator, and to thereby prevent the wavelength-converted laser light from being absorbed by the solid-state laser medium and to obtain wavelength-converted laser light with good efficiency.
FIG. 1 is a diagram showing the configuration of a resonator in a solid-state laser generator with intracavity wavelength conversion for obtaining the third harmonic (Japanese Laid-open Patent Publication No. 2006-156677). The resonator of a conventional solid-state laser generator has a configuration in which a Q-switch 103a, a solid-state laser medium 101a for amplifying fundamental laser light 109, a Q-switch 103b, a Q-switch 103c, a solid-state laser medium 101b for amplifying the fundamental laser light 109, and a Q-switch 103d are arranged in sequence in a single row between a flat mirror 104 as a resonance mirror, and a flat mirror 108 for redirecting the optical axis of the resonator, as shown in FIG. 1. The Q-switches and solid-state laser media are disposed so that the length of the resonator composed of the Q-switches 103a and 103b and the solid-state laser medium 101a, and the length of the resonator composed of the Q-switches 103c and 103d and the solid-state laser medium 101b are equal to each other.
The flat mirror 107 used for separating laser light is disposed in the path of the fundamental laser light 109 that has been reflected by the flat mirror 108, and a flat mirror 105 as a resonance mirror is disposed in the travel direction of the fundamental laser light 109 reflected by the flat mirror 107. A lens 106a is disposed between the flat mirror 108 and the flat mirror 105. Disposed on the optical axis of the laser between the flat mirror 107 and the flat mirror 105 are a nonlinear optical crystal 102a that converts fundamental laser light 109 to second-harmonic laser light 110, a nonlinear optical crystal 102b that optically mixes the fundamental laser light 109 and the second-harmonic laser light 110 and converts the mixed light to third-harmonic laser light 111, and a lens 106b. The flat mirror 105 has an incidence angle of 0° and is highly reflective with respect to the fundamental wave and the second harmonic. The lenses 106a and 106b are antireflection lenses with respect to the fundamental wave and the second harmonic. The flat mirror 107 has an incidence angle of 45°, is highly reflective with respect to the fundamental wave, and is antireflective with respect to the third harmonic.
Here, the flat mirror 107 may have a dielectric multilayer film that is highly reflective with respect to the fundamental laser light 109 and is antireflective with respect to third-harmonic laser light 111 produced as output, or may be a film that is antireflective or highly reflective with respect to second-harmonic laser light 110 as demanded by the application; and there may also be cases in which reflectivity is not specified.
However, the prior art described above has the following problems. When harmonics of two or more stages are obtained using intracavity wavelength conversion, the harmonic component obtained in an intermediate stage, i.e., the second-harmonic laser light described in Japanese Laid-open Patent Publication No. 2006-156677, is used only once in the process of conversion to higher-order harmonics depending on the purpose. There is therefore a problem in that the conversion efficiency does not become high even were the configuration to have intracavity wavelength conversion.